Effects Of Nitrogen Addition On Fine Root Decomposition Of Planted Chinese Pine Forest

Fine root decomposition is a necessary part of the material and energy cycle in the ecosystem.The nutrient elements such as carbon?C?,nitrogen?N?and phosphorus?P?released during the decomposition process play an important role in improving soil fertility,promoting plant growth and improving nutrient use efficiency.Fine root decomposition is carried out under the action of biological and non-biological,the rate of decomposition and dynamic process of nutrient release are affected by many factors.The current increasing N deposition has become a global environmental problem,and understanding the responses of forest ecosystems material and energy cycles to N deposition is important for maintaining forest ecosystem balance and global ecological health.The response of the fine root decomposition to the N addition has become a hotspot in this field,which is valuable for revealing the response mechanism of ecosystem energy cycle to the environmental changes.This research focused on three diameter class?0-0.5mm?0.5-1?1-2mm?fine root of Chinese pine?Pinus tabuliformis Carrière?,the specific design of N added as follows: 0?3?6?9g·N·m^-2·yr^-1,using the mesh bag and periodic sampling method to research the changes of C,N,P,soluble sugar,starch,cellulose,lignin contents.The main conclusions are as follows:N addition will first changed the nutrient and component contents of the fine root,and the responses of different nutrient elements and components to N addition will vary.Contents of C,N and P in the fine root had an increasing first and then decreasing trend with the N addition improvement,and the highest values at the addition level of 3 g·N·m^-2·yr^-1.It meanings that low concentration of N addition would promote the nutrient uptake of fine roots and would inhibited at high concentrations.Fine root decomposition rate had a rapid phase?within 60 days?and a slow phase?after 60 days?within 300 days.The dry matter loss of fine root during the decomposition process had no significant response to N addition.The release patterns of C,N and P were different during the process of fine root decomposition.C and N contents were first enriched,then released rapidly and finally remained stable or slow do wn within 300 days,P content had a stable release pattern within300 days.The release patterns of nutrient element between the different root diameter classes were also different.Fine root nutrient element remainings were consistent with their initial nutrient element contents,which mean that the release of the elements is related to the initialnutrient element content.N addition had promoted or inhibited effects on fine root nutrient element releases according to their root diameter classes and N addition level.Structural carbohydrates?cellulose,lignin and ash?and nonstructural carbohydrates?soluble sugar,starch?had different release pattern.Soluble sugar and starch contents decreased rapidly in the early stage of decomposition,cellulose,lignin and ash contents in the decomposition period were relatively stable or slightly decreased.High concentration of N addition(9 g·N·m^-2·yr^-1)would promote the decomposition rate of 0-0.5mm fine root soluble sugar and starch.The effects of N addition on the decomposition of fine root lignin and cellulose?promote or inhibit?were depended on their root diameter classes.There were many factors that affect fine root decomposition.Dry matter remaining at decomposition 300 days had significant negative correlation with initial C content,C/N,lignin/N,cellulose contents;and significant positive correlation with N,P,lignin and ash contents.Meanwhile these correlations were enhanced under low concentration N addition(3g·N·m^-2·yr^-1)conditions and these correlations are attenuated at high concentration(9g·N·m^-2·yr^-1).The content remaining of three elements,C,N and P,which were main elements of the plant component,are highly consistent with dry matter remaining during the decomposition process and are not affected by the N addition.But the significant correlation between lignin/N,lignin/P and dry matter remaining during the decomposition process were susceptible to N addition.